Polysilazanes and their derivatives are useful among other things, for the preparation of silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SiC), Si.sub.3 N.sub.4 /SiC alloys, Si.sub.3 N.sub.4 /carbon alloys, Si.sub.3 N.sub.4 /boron nitride alloys, and mixtures thereof. These ceramic materials can be used as structural materials, protective coatings, and electronic materials because of their hardness, strength, structural stability under extreme environmental conditions and their wide variety of electronic properties. In particular, these materials can be formed into ceramic fibers of value for reinforcement of composite materials. See, for example, (a) Department of Defense Proceedings, Fourth Metal Matrix Composites Technical Conference, May 19-21, 1981, prepared for DOD Metal Matrix Composites Information Analysis Center; and (b) J. J. Brennan, "Program to Study SiC Fiber-Reinforced Matrix Composites", Annual Report to Dept. of Navy (Nov. 1980), Contract No. N00014-78-C--0503.
Historically, polysilazanes were first synthesized by Stock et al almost 60 years ago (see, e.g., Stock, A. and K. Somieski, Ber. Dtsch. Chem. Ges. 54:740 (1921)) via a simple ammonolysis technique (Scheme I). However, this ##STR1## approach usually produces mixtures of cyclomers where x is 3 to 5 that are obtained as the major products and small amounts of linear oligomers where y is less than or equal to about 10. Because of their low molecular weight, however, these linear oligosilazanes are too volatile to be used as preceramic materials.
In order to obtain higher molecular weight, nonvolatile materials, it was necessary to promote crosslinking reactions. In this manner, moderate molecular weight polysilazanes have been synthesized using a variety of techniques. See, e.g., Kruger, C. R. and E. G. Rochow, J. Polymer Sci. 2A:3179-3189 (1964). Rochow et al. discovered that ammonium chloride catalyzes crosslinking in simple oligodimethylsilazanes to form polysilazanes (Scheme II) which ##STR2## were proposed to contain cyclic monomer units crosslinked through nitrogen as suggested by the structure ##STR3##
The Penn et al. work follows up on U.S. Pat. No. 3,853,567 to Verbeek and U.S. Pat. No. 3,892,583 to Winter et al., wherein a high temperature elimination/condensation reaction was shown to lead to soluble, highly crosslinked polymers as shown in Scheme III. Pyrolysis at high temperatures provides ceramic ##STR4## yields of 60% with a mixture of Si.sub.3 N.sub.4 and SiC ceramic materials.
A related crosslinking approach described, inter alia, in U.S. Pat. Nos. 4,312,970, 4,340,619, 4,535,007 and 4,543,344 begins with the preparation of tractable polysilazanes having Me.sub.3 Si groups in the polymer backbone (Scheme IV) with the highest molecular weights reported in the available literature, i.e., about Mw.about.15,000 D and Mz.about.39,000 D: ##STR5## Ceramic yields obtained from pyrolysis of this polymer are on the order of 45-55% with compositions of 96% Si.sub.3 N.sub.4, 2% carbon and 2% oxygen after curing.
U.S. Pat. No. 4,482,669 to Seyferth et al. discloses that it is possible to crosslink low molecular weight cyclic oligomers containing Si-H bonds adjacent to N-H bonds via the following reaction: ##STR6## The NH bond is catalytically activated by the strong base in this reaction. This type of crosslinking generates two-dimensional polymers, the solubility of which is limited by their sheet-like character. Ceramic yields of these materials are often quite high, up to about 86%, and typically provides Si.sub.3 N.sub.4, SiC and carbon in a mole ratio of 0.88:1.27:0.75. If the pyrolysis is carried out in an NH.sub.3 atmosphere, then the only product is Si.sub.3 N.sub.4 with the other products remaining as slight impurities.
Zoeckler and Laine in J. Org. Chem. (1983) 48:2539-2541 describe the catalytic activation of the Si-N bond and in particular the ring opening of octamethylcyclotetrasilazane and polymerization of the ring-opened intermediate. Chain termination is effected by introducing [CH.sub.3).sub.3 Si].sub.2 NH as a coreactant giving rise to polymers (CH.sub.3).sub.3 Si(CH.sub.3).sub.2 ].sub.n -NHSi(CH.sub.3).sub.3 where n may be 1 to 12 or more depending upon the ratio of the chain terminator to the cyclic silazane. The catalyst used was Ru.sub.3 (CO).sub.12. Other publications are as follows W. Fink, Helv. Chem Acta., 49:P1408 (1966); Belgian Patent 665774 (1965); Netherlands Patent 6,507,996 (1965); D. Y. Zhinkis et al., Rus. Chem. Rev., 49:2814 (1980); K. A. Andrianov et al., Dok Akad. Nauk SSSR, 227:352 (1976); Dok Akad. Nauk. SSSR 223:347 (1975); L. H. Sommer et al., JACS 91:7061 (1969); L. H. Sommer, J. Org. Chem. 32:2470 (1967); L. H. Sommer et al., JACS 89:5797 (1967).
In general, control of the polysilazane molecular weight, structural composition and viscoelastic properties plays a considerable role in determining the tractability (solubility, meltability or. malleability) of the polymer, the processability during. fabrication of fibers, shaped articles, etc., the ceramic yield, and the selectivity for specific ceramic products. In particular, the tractability plays a major role in how useful the polymer is as a binder, or for forming shapes, coatings, spinning fibers and the like. The more crosslinked a polymer is, the less control one has of its viscoelastic properties. Thus, highly crosslinked, low molecular weight polymers that are rigid materials or gels are not particularly useful for spinning fibers or as binders because they lack the required flexibility, viscosity and tenacity. By contrast, high molecular weight, flexible polymers as provided by applicants are extremely important. Such polymers represent a significant advance in the art, as they provide the flexibility and tenacity necessary in the fiber-spinning process and enhance the overall tensile strength of the spun fibers. In addition, the viscosities and the softening and melting points of the novel polymers may play a key role in binder applications, and in injection-molding processes in particular.
The parent application hereto, U.S. application Ser. No. 012,874, filed Dec. 1, 1986, describes the preparation of such high molecular weight, substantially linear polysilazanes. The disclosure of that application is hereby incorporated by reference in its entirety, as that application describes in some detail various compounds, methods and uses relevant to the present invention but not explicitly addressed herein.
The present application is directed to a subset of the compositions and methods described and claimed in Ser. No. 012,874. Specifically, the present application is directed to silazanes and polysilazanes that include at least one cyclomeric silazane unit in the molecular structure.
As discussed above, several routes to polymers containing the monomeric units -[MeSiHNH]- have been developed. Ammonolysis of MeSiHCl.sub.2 generates the low molecular weight cyclomer 1-cyclomethylsilazane (CMS) ##STR7## which, as a non-viscous liquid that gives ceramic yields on the order of 20 wt. %, is impractical as a ceramic precursor. Arai et al. (see U.S. Pat. No. 4,659,850) have demonstrated how a modification of the ammonolysis process results in a higher molecular weight species The Arai et al. group reacted dichloromethylsilane with ammonia in the presence of pyridine at 80.degree. C. The Lewis base complexes with the chlorosilane and causes the formation of a linear-cyclomer copolymer having trisilylated nitrogen bridges (M.sub.n =1100-1800 D, ceramic yield about 44 wt %). Similarly, Matsumoto et al. (Japan Patent Publication [Kokai] No. 61-72607) disclose reaction of a dihalosilane with ammonia which is stated to give relatively high molecular weight, highly viscous polysilazanes.
The Seyferth et al. polymers (U.S. Pat. No. 4,482,669, cited supra) formed by dehydrocyclodimerizing CMS oligomers, are, as noted above, very rigid structures. This rigidity, although responsible in part for the high ceramic yields obtained upon pyrolysis, prevents the polymer from having softening or melting points. Such polymers with an M.sub.n over about 2500 D are brittle, intractable gels. No liquid polymers, even of low molecular weight, have been formed by this KH catalysis method.
The presently claimed compounds are believed to include the first reported polymers of [MeSiHNH].sub.n that are either liquid or have a softening or melting point, indicating higher structural flexibility and perhaps higher linearity as well. Like the parent application hereto, the present disclosure demonstrates how transition metal catalysis may be used for modifying the characteristics of inorganic polymers and, specifically, how control over polymer properties, pyrolysis results and the final ceramic compositions can follow directly from the selection of the precursor, the chosen chemical method and the reaction conditions.